Condensers are widely used in the manufacturing, chemical and energy industry. The air-cooled condenser is a special type of condenser, which generally operates under a vacuum. First of all we shall describe the physical processes that take place in air-cooled condensers, to make sure that the operation of the air-cooled condenser according to the invention is understood.
The description of physical processes and of the prior art apply to power plant steam condensers and to condensing steam, but of course the invention is not restricted to this type of condenser: they can also be used as applicable in other places and for other vaporous mediums where air-cooled condensers are required.
Air-cooled steam condensers generally consist of a large number of tubes connected in parallel which are densely finned on the air side. The processes taking place in the parallel tubes are principally identical, so it suffices to describe the processes taking place in a single tube.
FIG. 1 shows a schematical cross-sectional view of a known air-cooled condenser comprising a distributing chamber 14, a condensate collecting chamber 16 arranged on a lower level, and these sloping connecting parallel coupled condenser tubes 1 of which only one is shown.
The cross-section of the condenser tubes 1 can be different, and in practice generally condenser tubes 1 with round, elliptical or flat, horse-race track shaped cross-section are used. Inside the condenser tube 1, the condensing steam flows in the direction of arrow 2, and outside the condenser tube 1, perpendicular to the axis thereof, the cooling air flows in the direction of arrows 3.
Since the steam condensing in the condenser tube 1 has a very high heat transfer coefficient, which may be as high as 23.260 W/m.sup.2 K, and the air side heat transfer coefficient is low, between 58 and 81 W/m.sup.2 K, it is advisable to increase the air-side surface in order to improve the efficiency of heat exchange, which is practically implemented by fins 4.
From the direction of arrow 2, not only pure steam enters the condenser tube 1, but also a very low quantity of non-condensable gases, mainly air. One part of the non-condensable gases, as volatile alkalizers and dissociation products, are carried by the steam, while the larger part gets into the steam as a result of leaks in the technological system. In the case of an appropriately implemented and maintained steam turbine, the amount of non-condensable gases--mainly air--entering the condenser with the steam is 0.005 to 0.01% by weight.
Although this quantity in relation to the steam is very low, it becomes obvious later on that the operation of the condenser is very much influenced by the presence of non-condensable gases.
The condensate of the steam and the non-condensable gases must be removed continuously. A pipe 6 and a condensate pump 10 serves to discharge condensate 5 from the condensate collecting chamber 16, while mixture 7 of the non-condensable gases and some remaining steam leaves through an air extraction pipe 8 towards a vacuum pump 9.
In the course of condensation, the change in important physical characteristics, i.e. in the partial pressure of the air, in the steam space under-cooling, and in the steam-side heat transfer coefficient can be neglected as long as 97 to 99% of the steam is not condensed. The only exceptions from this rule are the flow volume and velocity of the steam-air mixture 7, which are inversely proportional with the volume of the condensed steam. Thus for example if 97% of the steam is condensed, the flow volume and the velocity are only 3% of the values at the entry point.
However, in the condensation of the remaining 3%, but especially in that of the last 0.5% of steam, due to the presence of non-condensable gases, significant changes can be experienced in the various parameters, as can be seen in the following table.
______________________________________ Remaining steam 3% 0.6% 0.06% 0.01% volume partial pressure of 24 Pa 120 Pa 1200 Pa 5000 Pa air/non-condensable gases under-cooling of the 0.04.degree. C. 0.2.degree. C. 2.degree. C. 10.degree. C. condensation space decrease of steam- 10% 43% 82% 82% side heat transfer coefficient volume of flowing 3% 0.625% 0.065% 0.015% steam-air mixture ______________________________________
It can be seen that in the condensation of the remaining 3% of the steam the partial pressure of the air increases dramatically, and as a result, condensation temperature drops, or in other words, the under-cooling of the condensation space increases. Due to the increase in the air concentration, at the end of the condensation, the steam-side heat transfer coefficient decreases substantially. The volume of flowing steam-air drops to a fraction of the entry value.
Due to the changes listed above, it is a usual practice to separate the condenser, as shown in FIG. 2, to a main condenser 11 in which 80 to 90% of the steam is condensed and to an after-cooler 15 (dephlegmator), in which a part of the remaining steam is condensed and mixture 7 is under-cooled. The main condenser 11 and the after-cooler 15 are connected by the condensate collecting chamber 16, which on the one hand guides the steam exiting from the main condenser 11 to the after-cooler 15, and on the other collects the condensate 5, draining it through the pipe 6 to the condense pump 10.
The structure of the main condenser 11 corresponds to the condenser tube 1 in FIG. 1, i.e. the steam and the condensate 5 flow downwards in the same direction, but in the after-cooler 15, the mixture 7 flows upwards, and the condensate 5 downwards, in counterflow to the mixture 7. This is necessary because--as shown above--at the end of the condensation process the under-cooling of the mixture 7 dramatically increases, and in the case of ambient temperatures below the freezing point, the under-cooling could be of such a rate that the temperature of the condensation space also drops to below the freezing point, and as a result the condensate 5 could freeze up. The frozen condensate 5 could block the path of air extraction, causing the drop-out of the relevant condenser tube from the condensation process, and in the worst case, the frozen condensate 5 could even crack the tube.
The arrangement according to FIG. 1 also entails the disadvantages that due to the under-cooling of the steam space the temperature of the condensate 5 is lower than the theoretical condensation temperature, and when this condensate 5 is returned to the steam turbine cycle, it deteriorates the thermal efficiency of the system. A further undesirable effect is that due to the higher partial pressure of air and as a result of the under-cooling of the condensate 5, the latter absorbs a higher than permissible volume of oxygen, which could cause corrosion and require degassing prior to returning to the cycle.
The counterflow after-cooler 15 intends to reduce or eliminate these disadvantages, by making sure that the steam flowing in the opposite direction heats up the condensate 5.
The processes described so far arise when in the main condenser 11 and in the after-cooler 15 the steam-air mixture 7 flows towards the air extraction pipe 8 of the after-cooler 15. In the main condenser 11 this precondition is practically satisfied. If the condenser is dimensioned in a way that the steam velocity is 50 to 80 m/s at the entrance point, then assuming 95% condensation, at the exit of the main condenser 11 the steam velocity will be 2.5 to 4 m/s, which is just enough to make sure that the steam-air mixture 7 definitely flows in the direction of the exit.
In the after-cooler 15, however, this is not the case. Assuming that in the after-cooler 15 for the condensation of a remaining 5% steam, 10% of the tubes fitted into the main condenser 11 are installed, i.e. the flow cross-section drops to 1/10, the velocity at the entrance of the after-cooler 15 will be 25 to 40 m/s, but at the air extraction pipe 8 it will only be 0.16 to 0.25 m/s. To make sure that an excessive quantity of steam does not escape with the extracted air, and so that the application of a vacuum pump with an excessively large capacity is avoided, the after-cooler 15 is generally dimensioned in a way that at the air extraction pipe 8 the volume of the steam-air mixture 7 is only 0.03 to 0.04% of the entry volume, and that the air content of the extracted mixture 7 is 25 to 30% which occurs when the under-cooling of the steam-air mixture 7 is 4.degree. to 5.degree. C.
It is shown that the correct arrangement and dimensioning of the after-cooler 15 is an extremely difficult task. If for example a steam of low air content enters the after-cooler 15 at a high velocity, it reaches the air extraction pipe 8 as a result of the vortex flow and dilutes the mixture 7 to be extracted. The vacuum pump dimensioned for delivering a constant volume of air is then unable to remove all the air coming to the condenser, and so it accumulates first in the after-cooler 15 and then later in the main condenser 11 as well. The increasing air concentration dramatically increases the under-cooling of the steam space, and deteriorates the heat transfer coefficient, which entails a reduction in the heat dissipation of the condenser and may also cause a frost risk in cold weather. Since at the air extraction pipe 8 only extremely low volumes are flowing, fresh steam coming to this point even in a small volume could lead to the detrimental effects above.
Consequently, in the case of a correctly designed after-cooler, there should be no drastic drop of velocity between the inlet and extract points.
A correctly designed main condenser and after-cooler must also meet another requirement, namely that in the direction of the cooling air flow there should be only one row of finned tubes.
This is important because in the case of several tube rows, the tube row on the entry side of the cooling air receives much more cooling than the other tube rows, and so it has steam flowing in at both ends. The top end is the normal steam entry point, and the bottom end takes steam from the tubes of other rows via the common condensate collecting chamber.
As a result of this phenomenon, from the first, and eventually from the next tube row(s) the non-condensable gases are unable to escape, and stagnating air plugs develop. The length of these air plugs decreases gradually from the first tube row towards the next tube rows exposed to increasingly higher cooling air temperatures. In the stagnating zone filled up with air, the heat dissipation decreases and in a cold weather, frost risk may prevail. In order to eliminate these detrimental effects, air-cooled condensers with a single tube row are used. To make sure that a sufficient steam side cross section is available, an appropriate number of air-side fins can be installed and the air-side flow resistance is as low as possible, in practice generally flat finned tubes with horse-race track shaped cross-section are used.